Conventionally, image forming apparatuses including copiers, printers, and facsimiles, and others including post-processing apparatuses and bookbinding machines are equipped with punching devices to produce, for example, filing holes in sheets, such as paper sheets, delivered from them. In recent years, high-speed processing techniques have been developed for processing sheets delivered from image forming apparatuses and, in order to meet the latest developments, there are increasing demands for punching devices with high punching speed and high efficiency.
In most punching devices, a punching member, driven by a drive motor via a cam mechanism, is moved back and forth in the punching direction so that a sheet is punched after it is conveyed to a predetermined position. In this case, if a sheet is punched without a halt while it is conveyed, the punching member, after punching a hole, can come into strong contact with the edge of the punched hole as it is come out of the punched hole, leading to damage to the punched hole, deterioration in its quality, or paper jam.
Japanese Unexamined Patent Publication (Kokai) No. 2008-173734 describes a punching device in which a punching unit and a die unit are conveyed together with the sheet at the same speed while punching is performed. The punching unit and the die unit in this punching device are moved back and forth in the sheet conveyance direction by a crank mechanism that can move in the width direction of the sheet.
Japanese Unexamined Patent Publication (Kokai) No. 2013-43224 proposes a punching device that has a simpler structure and control mechanism to perform punching while conveying a punching unit and a die unit together with the sheet at the same speed, as in the above device. In this punching device, a punching unit having a punching member and a die is moved back and forth in the sheet conveyance direction by a cylindrical cam mounted on a horizontal rotary shaft while the punching member punches a hole in a sheet as it is moved back and forth by a bulging cam mounted coaxially on the shaft that supports the cylindrical cam.